A repeater is an automatic radio-relay station, usually located on a mountain top, tall building, or radio tower. It allows communication between two or more bases, mobile or portable stations that are unable to communicate directly with each other due to distance or obstructions between them.
The repeater receives on one radio frequency (the "input" frequency), demodulates the signal, and simultaneously re-transmits the information on its "output" frequency. All stations using the repeater transmit on the repeater's input frequency and receive on its output frequency. Since the repeater is usually located at an elevation higher than the other radios using it, their range is greatly extended.
Because the transmitter and receiver are on at the same time, isolation must exist to keep the repeater's own transmitter from degrading the repeater receiver. If the repeater transmitter and receiver are not isolated well, the repeater's own transmitter desensitizes the repeater receiver. The problem is similar to being at a rock concert and not being able to hear the weak signal of a conversation over the much stronger signal of the band.
In general, isolating the receiver from the transmitter is made easier by maximizing, as much as possible, the separation between input and output frequencies.
When operating through a repeater, mobile stations must transmit on a different frequency than the repeater output. Although the repeater site must be capable of simultaneous reception and transmission (on two different frequencies), mobile stations can operate in one mode at a time, alternating between receiving and transmitting; so, mobile stations do not need the bulky, and costly filters required at a repeater site. Mobile stations may have an option to select a "talk around" mode to transmit and receive on the same frequency; this is sometimes used for local communication within a range of the mobile units.
In looking at records of old systems, examples of cross-band commercial systems were found in every U.S. radio service where regulations allowed them. In California, specific systems using cross-band repeaters have existed at least since the 1960s. Historic examples of cross-band systems include:
In commercial systems, manufacturers stopped making cross-band mobile radio equipment with acceptable specifications for public safety systems in the early 1980s. At the time, some systems were dismantled because new radio equipment was not available. Sporadic E ionospheric ducting can make the 46 MHz and below frequencies unworkable in summer.
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Thanks for Visiting. A few things to remember;
1. You MAY NOT transmit on any of the Public Service freqs (police, fire, etc) no matter what your radio can do. It is illegal and dangerous.
2. You MAY NOT transmit in the amateur bands unless you have a license.
3 These lists come from many sources and I have no claim on them or guarantee they are active or on the air.
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